21 A, 600 V AC — what it handles
The A9-30-10-51: Rated 21 A continuous at 600 V AC, this 3-pole contactor switches motor loads up to that current in across-the-line starting — the 21 A figure is the thermal continuous rating, not the locked-rotor or inrush capability. The 480 V AC control coil means the pickup and dropout voltage must match the control transformer or PLC output driving it — a 120 V or 24 V coil version is a different order code.
Surface mount — panel integration
Surface-mount, not DIN-rail snap-on — the contactor bolts to a backplate or sub-panel. Plan for screw-termination clearance and wire-bend radius around the 1 NO auxiliary contact block.